1. Field of the Invention
The present invention relates to pneumatic conveying systems.
2. The Prior Art
Flowable particulate materials may be pneumatically conveyed in a conveying conduit. Such a conduit may, however, at times become clogged if accumulations of the particulate material occur, a possibility which can not be totally avoided.
To overcome this problem it has been proposed in German Allowed Application AS No. 2,305,030 to provide a secondary gas conduit which communicates with the conveying conduit at intervals, via release valves. When pressure decreases in a section of the conveying conduit because of clogging due to material-accumulation, the release valve in this section will automatically open since the pressure in the conveying conduit is now lower than that in the secondary conduit, and a burst of gas will be admitted into the conveying conduit to clear the obstruction.
A problem experienced with the prior art constructions is that the known release valves do not close tightly enough to prevent the conveyed material--especially if the same is in form of very small particles, e.g. in pulverulent form--from entering into the secondary conduit. This is highly undesirable, but heretofore unavoidable because the known release valves must operate at very small pressure differentials of about 1000 mm water column or less. Evidently, irrespective of the type of release valve used (e.g. ball valves, plate valves, or the like) a low response pressure means that the valve will have a correspondingly low closing pressure which will permit conveyed material to penetrate into the secondary conduit, especially after the valve has just been operated and has then returned to its closed position.
Consideration has already been given to use release valves which are constructed analogous to the valves used on vehicle tires. However, these valves--although able to prevent the entry of dust-like particles of conveyed material--have been found unsuitable for the type of application here under discussion, because they open only at pressure differentials which are substantially in excess of the ones at which gas must be permitted to enter the conveying conduit from the secondary conduit in order for the system to function satisfactorily.
A safety relief valve is known from German Pat. No. 758,917 which operates at a low pressure differential but has a high closing pressure nevertheless. Despite the fact that this valve thus meets the two above enumerated requirements, it is not suitable for use in the environment with which the invention is concerned. The reason for this is that the movable components of the valve, e.g. the valve spring and the valve-member guide, must be located in the downstream portion of the valve housing, as considered with reference to the direction of gas flow through the valve. If the valve were to be used in a pneumatic conveying system, this housing portion would have to face toward and communicate with the conveying conduit; the result would be rapid clogging of the valve by the conveyed material.